Height position detector for work held on chuck table

ABSTRACT

A height position detector for detecting the height position of an upper surface of a work held on a chuck table, including: an annular spot forming part by which the spot shape of a laser beam oscillated by a laser beam oscillator is formed into an annular shape; a first beam splitter by which the laser beam with the spot shape formed into the annular shape is guided into a first path; a light condenser by which the laser beam guided into the first path is condensed to irradiate the work therewith; a second beam splitter for splitting the reflected light reflected by the work; a first light receiving element for receiving the reflected light transmitted through the second beam splitter; a second light receiving element for receiving the reflected light reflected by the second beam splitter; a light reception region restricting part for restricting the light reception region for the reflected light received by the second light receiving element; and a controller for determining the height position of the upper surface of the work on the basis of the ratio between the quantity of light received by the first light receiving element and the quantity of light received by the second light receiving element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a height position detector fordetecting the height position of an upper surface of a work, such as asemiconductor wafer, held on a chuck table provided in a machiningapparatus such as a laser beam machining apparatus.

2. Description of the Related Art

In a semiconductor device manufacturing process, a surface of asemiconductor wafer in a roughly circular disk-like shape is providedwith a plurality of regions demarcated by planned dividing lines calledstreets arranged in a grid pattern, and devices such as ICs and LSIs areformed in the thus demarcated regions. Then, the semiconductor wafer iscut along the streets, whereby the regions with the devices formedtherein are divided from each other, to manufacture the individualdevices. Similarly, an optical device wafer in which a gallium nitridecompound semiconductor and the like are stackedly formed on a surface ofa sapphire substrate is cut along planned dividing lines into individualoptical devices such as light emitting diodes and laser diodes, whichare widely utilized for electric apparatuses.

As a method for dividing the semiconductor wafer, the optical devicewafer or the like along the streets formed therein, a laser beammachining method has been attempted in which irradiation of the waferwith a pulsed laser beam is conducted by using a pulsed laser beamtransmissive to the wafer and by positioning a light condensing point inthe inside of the regions to be divided. In the dividing method usingthe laser beam machining method, a wafer is irradiated from one sidethereof with a pulsed laser beam transmissive to the wafer which has awavelength of, for example, 1064 nm while positioning the lightcondensing point in the inside of the wafer, so as to continuously forma denatured layer in the inside of the wafer along the streets, and thework is divided by exerting an external force along the planned dividinglines lowered in strength by the formation of the denatured layer (referto, for example, Japanese Patent No. 3408805).

However, where the plate-like work such as a semiconductor wafer hasundulation and has a dispersion of its thickness, the denatured layercannot be formed uniformly at a predetermined depth through theirradiation with a laser beam, due to a factor associated with therefractive index of the work. Therefore, in order to form the denaturedlayer uniformly at a predetermined depth in the inside of thesemiconductor wafer or the like, it is necessary to preliminarily detectthe projection-and-recess form of the region to be irradiated with thelaser beam, and to cause laser beam irradiation means to track theprojection-and-recess form at the time of machining.

In order to solve the above-mentioned problem, the present applicant hasproposed a laser beam machining apparatus having height positiondetecting means by which a face-side surface (upper surface) of a workheld on a chuck table is irradiated with a visible laser beam, and,based on the quantity of light corresponding to the area of reflectionby the face-side surface (upper surface) of the work, the heightposition of the face-side surface (upper surface) of the work isdetected (refer to, for example, Japanese Patent Laid-open No.2007-152355).

In the height position detecting means disclosed in the just-mentionedlaid-open patent publication, in the case where the wafer as the work isformed of silicon, the visible laser beam is not transmitted through thework and, therefore, the quantity of light corresponding to the area ofreflection by the face-side surface (upper surface) of the work can bemeasured accurately. However, in the case where the wafer is formed ofsapphire or quartz having transparency to the laser beam, the laser beamis not only reflected by the face-side surface (upper surface) of thework but also reflected by the back-side surface (lower surface) of thework, so that it is impossible to measure only the quantity of the lightreflected by the face-side surface (upper surface) of the work.Therefore, it is impossible, by the height position detecting meansdisclosed in the laid-open patent publication, to detect the heightposition of a work formed from a material having a transparent property.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aheight position detector for securely detecting the height position ofan upper surface of a work held by a chuck table even in the case wherethe work is formed from a material having a transparent property.

In accordance with an aspect of the present invention, there is provideda height position detector for a work held on a chuck table, fordetecting the height position of an upper surface of a work held on achuck table, including: laser beam oscillating means for oscillating alaser beam; annular spot forming means by which a spot of the laser beamoscillated by the laser beam oscillating means is formed into an annularshape; a first beam splitter by which the laser beam with the spotformed into the annular shape by the annular spot forming means isguided into a first path; a light condenser by which the laser beamguided into the first path is condensed so as to irradiate the work heldon the chuck table therewith; a pinhole mask disposed in a second pathinto which reflected light reflected by the work held on the chuck tableis split by the first beam splitter; a second beam splitter by which thereflected light having passed through the pinhole mask is split into athird path and a fourth path; a first light receiving element forreceiving the reflected light split into the third path by the secondbeam splitter; a second light receiving element for receiving thereflected light split into the fourth path by the second beam splitter;light reception region restricting means which is disposed in the fourthpath and which restricts a reflected light reception region where thereflected light is received by the second light receiving element; andcontrol means by which the height position of the upper surface of thework held on the chuck table is determined based on the ratio betweenthe quantity of light received by the first light receiving element andthe quantity of light received by the second light receiving element.

The annular spot forming means includes a pair of conical lensesarranged in series at a predetermined interval along the laser beam.

The height position detector for a work held on a chuck table accordingto the present invention is configured as above, the laser beam with acircular spot shape oscillated from the laser beam oscillating means isformed into a laser beam with an annular spot shape by the annular spotforming means, and the work is irradiated with the laser beam having theannular spot shape. Therefore, the laser beam having the annular spotshape with which the work is irradiated is reflected by the uppersurface of the work while having the annular spot shape and, in the casewhere the work has a transparent property, the laser beam is reflectedalso by the lower surface of the work while having an annular spotshape. Then, the second reflected light with the annular spot shapewhich has been reflected by the lower surface of the work is interceptedby the pinhole mask, and the quantity of received light is detectedbased on the first reflected light with the annular spot shape which hasbeen reflected by the upper surface of the work and passed through thepinhole in the pinhole mask, so that the position of the upper surfaceof the work can be accurately detected even in the case where the workhas a transparent property.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser beam machining apparatusequipped with a height position detector for a work held on a chucktable which is configured according to the present invention;

FIG. 2 is a block diagram showing the configuration of the positionheight detector for a work held on a chuck table that is configuredaccording to the present invention;

FIG. 3 illustrates the condition where a laser beam with a circular spotshape is formed into an annular spot shape by annular spot forming meansconstituting the height position detector shown in FIG. 2;

FIG. 4 illustrates the condition where a work held on the chuck table isirradiated with a laser beam by the height position detector shown inFIG. 2;

FIG. 5 illustrates the condition where a part of reflected light splitby a first beam splitter constituting the height position detector shownin FIG. 2 is intercepted by a pinhole mask whereas another part passesthrough the pinhole mask;

FIGS. 6A and 6B illustrate the condition where works having differentthicknesses held on the chuck table are individually irradiated with alaser beam by the height position detector shown in FIG. 2;

FIG. 7 is a control map showing the relation between the ratio of avoltage (V1) outputted from a first light receiving element constitutingthe height position detector shown in FIG. 2 to a voltage (V2) outputtedfrom a second light receiving element constituting the detector, and thedistance from a light condenser to the upper surface of the work;

FIG. 8 is a block diagram showing control means constituting the heightposition detector shown in FIG. 2;

FIG. 9 is a perspective view of a semiconductor wafer as a work;

FIGS. 10A and 10B illustrate relations of the semiconductor wafer shownin FIG. 9 in the state of being held in a predetermined position on thechuck table of the laser beam machining apparatus shown in FIG. 1, withcoordinate positions;

FIG. 11 illustrates a height position detecting step carried out by theheight position detector provided in the laser beam machining apparatusshown in FIG. 1;

FIGS. 12A and 12B illustrate a machining step in which a denatured layeris formed in the semiconductor wafer shown in FIG. 9 by the laser beammachining apparatus shown in FIG. 1; and

FIG. 13 illustrates a machining step in the case where the work is largein thickness.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, a preferred embodiment of a height position detector for a workheld on a chuck table which is configured according to the presentinvention will be described more in detail below, referring to theattached drawings. FIG. 1 shows a perspective view of a laser beammachining apparatus as a machining apparatus equipped with a heightposition detector for a work held on a chuck table which is configuredaccording to the present invention. The laser beam machining apparatusshown in FIG. 1 includes: a stationary base 2; a chuck table mechanism 3for holding a work which is disposed on the stationary base 2 so as tobe movable in a machining feed direction indicated by arrow X; a laserbeam irradiation unit support mechanism 4 disposed on the stationarybase 2 so as to be movable in an indexing feed direction (Y-axisdirection) indicated by arrow Y which is orthogonal to the direction(X-axis direction) indicated by arrow X; and a laser beam irradiationunit 5 disposed on the laser beam unit support mechanism 4 so as to bemovable in a direction (Z-axis direction) indicated by arrow Z.

The chuck table mechanism 3 include: a pair of guide rails 31, 31disposed on the stationary base 2 in parallel to each other along themachining feed direction indicated by arrow X; a first slide block 32disposed on the guide rails 31, 31 so as to be movable in the machiningdirection (X-axis direction) indicated by arrow X; a second slide block33 disposed on the first slide block 32 so as to be movable in theindexing feed direction (Y-axis direction) indicated by arrow Y; a covertable 35 supported on the second slide block 33 by a hollow cylindricalmember 34; and a chuck table 36 as work holding means. The chuck table36 has a suction chuck 361 formed from a porous material, and a work,for example, a circular disk-like semiconductor wafer is held on thesuction chuck 361 by suction means (not shown). The chuck table 36 thusconfigured is rotated by a pulse motor (not shown) disposed in thehollow cylindrical member 34. Incidentally, the chuck table 36 isequipped with clamps 362 for fixing an annular frame which will bedescribed later.

The first slide block 32 is provided in its lower surface with a pair ofguided grooves 321, 321 in which to fit the pair of guide rails 31, 31,and is provided on its upper surface with a pair of guide rails 322, 322formed in parallel to each other along the indexing feed directionindicated by arrow Y. The first slide block 32 thus configured ismovable in the machining feed direction indicated by arrow X along thepair of guide rails 31, 31, with its guided grooves 321, 321 fitted overthe pair of guide rails 31, 31. The chuck table mechanism 3 in theembodiment shown in the figure is equipped with machining feeding means37 by which the first slide block 32 is moved in the machining feeddirection indicated by arrow X along the pair of guide rails 31, 31.

The machining feeding means 37 includes a male screw rod 371 disposed inparallel to and between the pair of guide rails 31 and 31, and a drivesource such as a pulse motor 372 for driving the male screw rod 371 torotate. Of the male screw rod 371, one end is rotatably supported on abearing block 373 fixed to the stationary base 2, and the other end ispower-transmittingly connected to an output shaft of the pulse motor372. Incidentally, the male screw rod 371 is in screw engagement with apenetrating female screw hole formed in a female screw block (not shown)projectingly provided at a lower surface of a central part of the firstslide block 32. Therefore, with the male screw rod 371 driven by thepulse motor 372 to rotate normally and reversely, the first slide block32 is moved in the machining feed direction (X-axis direction) indicatedby arrow X along the guide rails 31, 31.

The laser beam machining apparatus in the embodiment shown in the figureis provided with X-axis direction position detecting means 374 fordetecting the position in the X-axis direction of the chuck table 36.The X-axis direction position detecting means 374 includes a linearscale 374 a disposed along the guide rail 31, and a reading head 374 bdisposed on the first slide block 32 and moved along the linear scale374 a together with the first slide block 32. The reading head 374 b ofthe X-axis direction position detecting means 374, in the embodimentshown in the figure, sends a pulse signal containing one pulse per 1 μmto control means which will be described later. The control meansdescribed later counts the pulses in the pulse signal inputted thereto,thereby detecting the position in the X-axis direction of the chucktable 36.

The second slide block 33 is provided in its lower surface with a pairof guided grooves 331, 331 in which to fit the pair of guide rails 322,322 provided on the upper surface of the first slide block 32, and ismovable in the indexing feed direction (Y-axis direction) indicated byarrow Y, with its guided grooves 331, 331 fitted over the pair of guiderails 322, 322. The chuck table mechanism 3 in the embodiment shown inthe figure is provided with first indexing feeding means 38 for movingthe second slide block 33 in the indexing feed direction (Y-axisdirection) indicated by arrow Y along the pair of guide rails 322, 322provided on the first slide block 32. The first indexing feeding means38 includes a male screw rod 381 disposed in parallel to and between thepair of guide rails 322 and 322, and a drive source such as a pulsemotor 382 for driving the male screw rod 381 to rotate.

Of the male screw rod 381, one end is rotatably supported on a bearingblock 383 fixed to the upper surface of the first slide block 32, andthe other end is power-transmittingly connected to an output shaft ofthe pulse motor 382. Incidentally, the male screw rod 381 is in screwengagement with a penetrating female screw hole formed in a female screwblock (not shown) projectingly provided on a lower surface of a centralpart of the second slide block 33. Therefore, with the male screw rod381 driven by the pulse motor 382 to rotate normally and reversely, thesecond slide block 33 is moved in the indexing feed direction (Y-axisdirection) indicated by arrow Y along the guide rails 322, 322.

The laser beam machining apparatus in the embodiment shown in the figureis provided with Y-axis direction position detecting means 384 fordetecting the position in the Y-axis direction of the second slide block33. The Y-axis direction position detecting means 384 includes a linearscale 384 a disposed along the guide rail 322, and a reading head 384 bdisposed on the second slide block 33 and moved along the linear scale384 a together with the second slide block 33. The reading head 384 b ofthe Y-axis direction position detecting means 384, in the embodimentshown in the figure, sends a pulse signal containing one pulse per 1 μmto the control means which will be described later. The control meansdescribed later counts the pulses in the pulse signal inputted thereto,thereby detecting the position in the Y-axis direction of the chucktable 36.

The laser beam irradiation unit support mechanism 4 includes a pair ofguide rails 41, 41 disposed on the stationary base 2 in parallel to eachother along the indexing feed direction (Y-axis direction) indicated byarrow Y, and a movable support base 42 disposed on the guide rails 41,41 so as to be movable in the direction indicated by arrow Y. Themovable support base 42 includes a moving support part 421 movablydisposed on the guide rails 41, 41, and a mount part 422 attached to themoving support part 421. The mount part 422 is provided on its sidesurface with a pair of guide rails 423, 423 extending in the directionindicated by arrow Z and being parallel to each other. The laser beamirradiation unit support mechanism 4 in the embodiment shown in thefigure is provided with second indexing feeding means 43 for moving themovable support base 42 in the indexing feed direction (Y-axisdirection) indicated by arrow Y along the pair of guide rails 41, 41.

The second indexing feeding means 43 includes a male screw rod 431disposed in parallel to and between the pair of guide rails 41, 41, anda drive source such as a pulse motor 432 for driving the male screw rod431 to rotate. Of the male screw rod 431, one end is rotatably supportedon a bearing block (not shown) fixed to the stationary base 2, and theother end is power-transmittingly connected to an output shaft of thepulse motor 432. Incidentally, the male screw rod 431 is in screwengagement with a female screw hole formed in a female screw block (notshown) projectingly provided on a lower surface of a central part of themoving support part 421 constituting the movable support base 42.Therefore, with the male screw rod 431 driven by the pulse motor 432 torotate normally and reversely, the movable support base 42 is moved inthe indexing feed direction (Y-axis direction) indicated by arrow Yalong the guide rails 41, 41.

The laser beam irradiation unit 5 includes a unit holder 51, and laserbeam irradiation means 52 attached to the unit holder 51. The unitholder 51 is provided with a pair of guided grooves 511, 511 in which toslidably fit the pair of guide rails 423, 423 provided on the mount part422, and is supported so as to be movable in the direction (Z-axisdirection) indicated by arrow Z, with its guided grooves 511, 511 fittedover the guide rails 423, 423.

The laser beam irradiation unit 5 has light condensing point positionadjusting means 53 for moving the unit holder 51 in a focal pointadjusting direction (Z-axis direction) indicated by arrow Z along thepair of guide rails 423, 423. The light condensing point positionadjusting means 53 includes a male screw rod (not shown) disposedbetween the pair of guide rails 423, 423, and a drive source such as apulse motor 532 for driving the male screw rod to rotate. With the malescrew rod (not shown) driven by the pulse motor 532 to rotate normallyand reversely, the unit holder 51 and the laser beam irradiation means52 are moved in the focal point position adjusting direction (Z-axisdirection) indicated by arrow Z along the guide rails 423, 423.Incidentally, in the embodiment shown in the figure, with the pulsemotor 532 driven to rotate normally, the laser beam irradiation means 52is moved upwards, and, with the pulse motor 532 driven to rotatereversely, the laser beam irradiation means 52 is moved downwards.

The laser beam irradiation unit 5 has Z-axis direction positiondetecting means 55 for detecting the position in the Z-axis direction ofthe laser beam irradiation means 52. The Z-axis direction positiondetecting means 55 includes a linear scale 551 disposed in parallel tothe guide rails 423, 423, and a reading head 552 attached to the unitholder 51 and moved along the linear scale 551 together with the unitholder 51. The reading head 552 in the Z-axis direction positiondetecting means 55, in the embodiment shown in the figure, sends a pulsesignal containing one pulse per 1 μm to the control means which will bedescribed later.

The laser beam irradiation means 52 includes a hollow cylindrical casing521 disposed substantially horizontally. As shown in FIG. 2, in thecasing 521 there are provided machining pulsed laser beam oscillatingmeans 6, and a light condenser 7 by which a work held on the chuck table36 is irradiated with a machining pulsed laser beam oscillated by themachining pulsed laser beam oscillating means 6. The machining pulsedlaser beam oscillating means 6 oscillates a machining pulsed laser beamLB1 having such a wavelength as to be transmissive to the wafer servingas a work. As the machining pulsed laser beam oscillating means 6, therecan be used, for example, a YVO4 pulsed laser oscillator or YAG pulsedlaser oscillator for oscillating a machining pulsed laser beam LB1having a wavelength of 1064 nm. The light condenser 7 includes adeflecting mirror 71 by which the direction of the machining pulsedlaser beam LB1 oscillated from the machining pulsed laser beamoscillating means 6 is deflected toward the lower side in FIG. 2, and acondenser lens 72 for condensing the machining pulsed laser beam LB1deflected by the deflecting mirror 71.

Referring to FIG. 2 again, the laser beam machining apparatus in theembodiment shown in the figure has a height position detector 8 fordetecting the height position of an upper surface of the work held onthe chuck table. The height position detector 8 includes: inspectionlaser beam oscillating means 80 for oscillating an inspection laserbeam; a dichroic half-mirror 81 which is disposed between the machiningpulsed laser beam oscillating means 6 and the light condenser 7 and bywhich the inspection laser beam oscillated from the inspection laserbeam oscillating means 80 is split toward the light condenser 7; annularspot forming means 82 which is disposed between the dichroic half-mirror81 and the inspection laser beam oscillating means 80 and by which thespot shape (sectional shape) of the inspection laser beam oscillated bythe inspection laser beam oscillating means 80 is formed into an annularshape; and a first beam splitter 83 which is disposed between theannular spot forming means 82 and the dichroic half-mirror 81 and bywhich the inspection laser beam with its spot shape (sectional shape)formed into the annular shape by the annular spot forming means 82 isguided into a first path 83 a directed toward the dichroic half-mirror81.

As the inspection laser beam oscillating means 80, there can be used,for example, a He—Ne pulsed laser oscillator for oscillating aninspection laser beam LB2 a which has a frequency different from thefrequency of the machining pulsed laser beam oscillated from themachining laser beam oscillating means 6, for example, which has awavelength of 635 nm. The dichroic half-mirror 81 transmits themachining pulsed laser beam LB1 but reflects the inspection laser beamoscillated from the inspection laser beam oscillating means 80 towardsthe light condenser 7. The annular spot forming means 82, in theembodiment shown in the figure, includes a first conical lens 821 and asecond conical lens 822 which are arranged in series with each other ata predetermined interval along the inspection laser beam LB2 a.Incidentally, while the pair of the first conical lens 821 and thesecond conical lens 822 are arranged with their vertexes facing eachother in the embodiment shown in the figure, they may be arranged withtheir back surfaces facing each other or may be arranged in the state ofbeing oriented in the same direction.

The annular spot forming means 82 thus configured functions so that theinspection laser beam LB2 a with the circular spot shape which isoscillated by the inspection laser beam oscillating means 80 is formedinto a laser beam LB2 b having an annular spot shape. Incidentally, theannular spot forming means 82 may be a mask member provided with anannular hole. The first beam splitter 83 functions so that the laserbeam LB2 b with its spot shape formed into an annular shape by theannular spot forming means 82 is guided into the first path 83 adirected toward the dichroic half-mirror 81, and the reflected light(described later) split by the dichroic half-mirror 81 is guided into asecond path 83 b.

The height position detecting means 8 includes: a pinhole mask 84including a pinhole 841 disposed in the second path 83 b and operativeto restrict the passage of the reflected light having a diameter greaterthan a predetermined diameter; a second beam splitter 85 by which thereflected light having passed through the pinhole mask 84 is split intoa third path 85 a and a fourth path 85 b; a condenser lens 86 forcondensing 100% of the reflected light split into the third path 85 a bythe second beam splitter 85; and a first light receiving element 87 forreceiving the reflected light condensed by the condenser lens 86. Thefirst light receiving element 87 sends a voltage signal corresponding tothe quantity of received light to the control means which will bedescribed later. Incidentally, the pinhole 841 formed in the pinholemask 84 has a diameter set to 1 mm, for example.

In addition, the height position detecting means 8 includes a secondlight receiving element 88 for receiving the reflected light split intothe fourth path 85 b by the second beam splitter 85, and light receptionregion restricting means 89 for restricting the reception region for thereflected light received by the second light receiving element 88. Thelight reception region restricting means 89, in the embodiment shown inthe figure, includes a cylindrical lens 891 for condensing into aone-dimensional form the reflected light split into the fourth path 85 bby the second beam splitter 85, and a one-dimensional mask 892 by whichthe reflected light condensed into the one-dimensional form by thecylindrical lens 891 is restricted to unit length. The second lightreceiving element 88 for receiving the reflected light having passedthrough the one-dimensional mask 892 sends a voltage signalcorresponding to the quantity of received light to the control meanswhich will be described later.

The height position detecting means 8 is thus configured, and itsoperation will be described below. As shown in FIG. 3, the inspectionlaser beam LB2 a having a circular spot shape S1 which is oscillatedfrom the inspection laser beam oscillating means 80 is formed into theinspection laser beam LB2 b having an annular spot shape S2 by theannular spot forming means 82. Specifically, the annular spot formingmeans 82 functions so that the laser beam LB2 a having a diameter of 2mm is expanded into the annular laser beam LB2 b having, for example, anouter diameter (D1) of 10 mm and an inner diameter (D2) of 8 mm and,simultaneously, is formed into a parallel beam. As shown in FIG. 2, theinspection laser beam LB2 b formed into the annular spot shape S2 by theannular spot forming means 82 is guided into the first path 83 a by thefirst beam splitter 83, reaches the dichroic half-mirror 81, and isreflected by the dichroic half-mirror 81 toward the light condenser 7.The inspection laser beam LB2 b reflected toward the light condenser 7is deflected by the deflecting mirror 71 toward the lower side in FIG.2, like the machining pulsed laser beam LB1, and is condensed by thecondenser leans 72.

In the case of irradiating the upper surface of the work W held on thechuck table 36 with the inspection laser beam LB2 b formed into theannular spot shape S2 as above-mentioned, the light condensing pointposition adjusting means 53 is so operated that the light condensingpoint Pb is located on the upstream side (upper side) in the laser beamirradiation direction relative to the upper surface of the work W, asshown in FIG. 4. As a result, the inspection laser beam LB2 b formedinto the annular spot shape S2 is radiated onto the upper surface of thework W held on the chuck table 36 in an annular spot shape S3, and isreflected in the size of the annular spot shape S3 (first reflectedlight). In this instance, in the case where the work W is formed fromsapphire or quartz having a transparent property, the inspection laserbeam LB2 b is transmitted through the work W to reach the lower surfaceof the work W, and is reflected in the size of an annular spot shape S4(second reflected light).

The first reflected light having the annular spot shape S3 which is thusreflected by the upper surface of the work W and the second reflectedlight having the annular spot shape S4 which is thus reflected by thelower surface of the work W reach the first beam splitter 83 through thecondenser lens 72, the deflecting mirror 71, and the dichroichalf-mirror 81. As shown in FIG. 5, the first reflected light LB2 chaving the annular spot shape S3 and the second reflected light LB2 dhaving the annular spot shape S4 which reach the first beam splitter 83are split into the second path 83 b by the first beam splitter 83, toreach the pinhole mask 84. The pinhole 841 formed in the pinhole mask84, in the embodiment shown in the figure, has a diameter set to 1 mm,for example, so that the first reflected light LB2 c having the annularspot shape S3 is permitted to pass through the pinhole 841, whereas thesecond reflected light LB2 d having the annular spot shape S4 isintercepted by the pinhole mask 84.

Incidentally, the diameter of the pinhole 841 formed in the pinhole mask84 is set by taking the thickness of the work W, the position of thelight condensing point Pb and the like into account so that the firstreflected light LB2 c having the annular spot shape S3 is permitted topass through the pinhole 841 whereas the second reflected light LB2 dhaving the annular spot shape S4 is intercepted by the pinhole mask 84.Thus, the second reflected light LB2 d having the annular spot shape S4which has been reflected by the lower surface of the work W isintercepted by the pinhole mask 84, and only the first reflected lightLB2 c having the annular spot shape S3 which has been reflected by theupper surface of the work W is permitted to pass through the pinhole 841in the pinhole mask 84.

The first reflected light LB2 c with the annular spot shape S3 reflectedby the upper surface of the work W and permitted to pass through thepinhole 841 of the pinhole mask 84 as above-mentioned is split by thesecond beam splitter 85 into the third path 85 a and the fourth path 85b, as shown in FIG. 2. The first reflected light LB2 c with the annularspot shape S3 split into the third path 85 a is entirely (100%)condensed by the condenser lens 86 and received by the first lightreceiving element 87. Then, the first light receiving element 87 sends avoltage signal corresponding to the quantity of received light to thecontrol means which will be described later. On the other hand, thesecond reflected light LB2 d with the annular spot shape S4 dispersedinto the fourth path 85 b is condensed into a one-dimensional form bythe cylindrical lens 891 of the light reception region restricting means89, is restricted to a predetermined unit length by the one-dimensionalmask 892, and is received by the second light receiving element 88.Then, the second light receiving element 88 sends a voltage signalcorresponding to the quantity of received light to the control meanswhich will be described later.

Here, the quantities of the first reflected light LB2 c having theannular spot shape S3 received respectively by the first light receivingelement 87 and the second light receiving element 88 will be described.The first reflected light LB2 c having the annular spot shape S3received by the first light receiving element 87 is constant in quantitybecause it is entirely (100%) received by the condenser lens 86, so thatthe voltage (V1) outputted from the first light receiving element 87 isconstant (for example, 10 V). On the other hand, the first reflectedlight LB2 c having the annular spot shape S3 received by the secondlight receiving element 88 is condensed into a one-dimensional form bythe cylindrical lens 891 and restricted to a predetermined unit lengthby the one-dimensional mask 892, before being received by the secondlight receiving element 88. Therefore, the quantity of light received bythe second light receiving element 88 varies depending on the distancefrom the condenser lens 72 of the light condenser 7 to the upper surfaceof the work W, namely, on the height position (thickness) of the work W,in the case where the upper surface of the work W is irradiated with theinspection laser beam LB2 b as shown in FIG. 4. Accordingly, the voltage(V2) outputted from the second light receiving element 88 variesdepending on the height position of the upper surface of the work Wirradiated with the inspection laser beam LB2 b.

For example, where the height position of the work W is higher (thethickness of the work W is greater) and the distance (H) from thecondenser lens 72 to the upper surface of the work W is smaller as shownin FIG. 6A, the inspection laser beam LB2 b is reflected in the shape ofthe annular spot S3 a with which the upper surface of the work W isirradiated. The reflected light is split into the third path 85 a andthe fourth path 85 b by the second beam splitter 85 as above-mentioned;in this case, the reflected light with the annular spot S3 a split intothe third path 85 a is entirely (100%) condensed by the condenser lens86, so that the whole quantity of the reflected light is received by thefirst light receiving element 87. On the other hand, the reflected lightwith the annular spot S3 a split into the fourth path 85 b by the secondbeam splitter 85 is condensed into a one-dimensional form by thecylindrical lens 891, so as to be substantially rectangular in section.The reflected light thus condensed into a substantially rectangularsectional shape is restricted to a predetermined unit length by theone-dimensional mask 892, so that a part of the reflected light splitinto the fourth path 85 b is received by the second light receivingelement 88. Therefore, the quantity of the reflected light received bythe second light receiving element 88 is smaller than the quantity oflight received by the first light receiving element 87.

Next, where the height position of the work W is lower (the thickness ofthe work W is smaller) and the distance (H) from the condenser lens 72to the upper surface of the work W is greater as shown in FIG. 6B, theinspection laser beam LB2 b is reflected in the shape of the annularspot S3 b with which the upper surface of the work W is irradiated. Thisannular spot S3 b is greater than the above-mentioned annular spot S3 a.The reflected light with the annular spot S3 b is split into the thirdpath 85 a and the fourth path 85 b by the second beam splitter 85 asabove-mentioned; in this case, the reflected light in the annular areaS3 b split into the third path 85 a is entirely (100%) condensed by thecondenser lens 86, so that the whole quantity of the reflected light isreceived by the first light receiving element 87.

On the other hand, the reflected light with the annular spot S3 b splitinto the fourth path 85 b by the second beam splitter 85 is condensedinto a one-dimensional form by the cylindrical lens 891, so as to besubstantially rectangular in section. The length of the major edge ofthe substantially rectangular shape is greater than that in the case ofthe annular spot S3 a, since the annular spot S3 b of the reflectedlight is greater than the annular spot S3 a. The reflected light thuscondensed to be substantially rectangular in section is cut to apredetermined length by the one-dimensional mask 892, and a part of thecondensed reflected light is received by the second light receivingelement 88. Therefore, the quantity of light received by the secondlight receiving element 88 is smaller than that in the case shown inFIG. 6A. Thus, the quantity of the reflected light received by thesecond light receiving element 88 is greater as the distance (H) fromthe condenser lens 72 to the upper surface of the work W is smaller,namely, as the height position of the work W is higher (the thickness ofthe work W is larger), and the quantity of the reflected light receivedby the second light receiving element 88 is smaller as the distance (H)from the condenser lens 72 to the upper surface of the work W is larger,namely, as the height position of the upper surface of the work W islower (the thickness of the work W is smaller).

Here, the relation between the ratio of the voltage (V1) outputted fromthe first light receiving element 87 to the voltage (V2) outputted fromthe second light receiving element 88 and the distance (H) from thecondenser lens 72 to the upper surface of the work W, namely, the heightposition of the work W, will be described referring to the control mapshown in FIG. 7. Incidentally, the axis of abscissas in FIG. 7represents the distance (H) from the condenser lens 72 to the uppersurface of the work W, and the axis of ordinates represents the ratio(V1/V2) of the voltage (V1) outputted from the first light receivingelement 87 to the voltage (V2) outputted from the second light receivingelement 88. In the example shown in FIG. 7, such a setting is made thatthe voltage ratio (V1/V2) is “1” where the distance (H) from thecondenser lens 72 to the upper surface of the work W is 30.0 mm and thatthe voltage ratio (V1/V2) is “10” where the distance (H) from thecondenser lens 72 to the upper surface of the work W is 30.6 mm.Therefore, the distance (H) from the condenser lens 72 to the uppersurface of the work W can be determined by determining the ratio (V1/V2)of the voltage (V1) outputted from the first light receiving element 87to the voltage (V2) outputted from the second light receiving element88, and collating the voltage ratio (V1/V2) with the control map shownin FIG. 7. Incidentally, the control map shown in FIG. 7 is stored in amemory of the control means which will be described later.

In use of the height position detecting means 8 configured as above, theinspection laser beam LB2 a having the circular spot shape S1 which isoscillated from the inspection laser beam oscillating means 80 is formedinto the inspection laser beam LB2 b having the annular spot shape S2 bythe annular spot forming means 82, and the work W is irradiated with theinspection laser beam LB2 b having the annular spot shape S2. Therefore,as shown in FIG. 4, the inspection laser beam LB2 b having the annularspot shape S2 with which the work W is irradiated is reflected in theannular spot shape S3 by the upper surface of the work W; in addition,where the work W has a transparent property, the inspection laser beamLB2 b is reflected in the annular spot shape S4 by the lower surface ofthe work W. The second reflected light LB2 b having the annular spotshape S4 reflected by the lower surface of the work W is intercepted bythe pinhole mask 84, and the quantity of light received is detectedbased on the first reflected light LB2 c of the annular spot shape S3which has been reflected by the upper surface of the work W and haspassed through the pinhole 841 in the pinhole mask 84. Therefore, theheight position of the upper surface of the work W can be accuratelydetected even where the work W has a transparent property.

Referring to FIG. 1 again, at a tip part of the casing 521 constitutingthe laser beam irradiation means 52, image pickup means 9 is disposedfor detecting a machining region to be subjected to laser beam machiningby use of the laser beam irradiation means 52. The image pickup means 9includes not only an ordinary image pickup device (CCD) for picking upan image by use of a visible beam but also IR illuminating means forilluminating the work with infrared rays, an optical system for catchingthe infrared rays radiated from the IR illuminating means, an imagepickup device (IR CCD) for outputting an electrical signal correspondingto the infrared rays caught by the optical system, and sends an imagesignal corresponding to the image thus picked up to the control meanswhich will be described later.

The laser beam machining apparatus in the embodiment shown in thefigures has the control means 10 shown in FIG. 8. The control means 10is comprised of a computer, including a central processor unit (CPU) 101for performing arithmetic processes according to a control program, aread only memory (ROM) 102 for storing the control program or the like,a readable and writable random access memory (RAM) 103 for storing theresults of arithmetic processes or the like, an input interface 104 andan output interface 105. The input interface 104 of the control means 10is supplied with detection signals from the X-axis direction positiondetecting means 374, the Y-axis direction position detecting means 384,the light condensing point position adjusting means 53, the first lightreceiving element 87, the second light receiving element 88, the imagepickup means 9, etc. Besides, control signals are outputted from theoutput interface 105 of the control means 10 to the pulse motor 372, thepulse motor 382, the pulse motor 432, the pulse motor 532, the machiningpulsed laser beam oscillating means 6, the inspection laser beamoscillating means 80, etc. Incidentally, the random access memory (RAM)103 includes a first storage region 103 a for storing the control mapshown in FIG. 7, a second storage region 103 b for storing design datafor the work which will be described later, a third storage region 103 cfor storing the height positions for the optical device wafer 10 whichwill be described later, and other storage regions.

The laser beam machining apparatus in the embodiment shown in thefigures is configured as above, and its operation will be describedbelow. FIG. 9 shows a perspective view of an optical device wafer 20 asthe work to be subjected to laser beam machining. The optical devicewafer 20 shown in FIG. 9 has a sapphire wafer, wherein a plurality ofregions are demarcated by a plurality of streets (planned dividinglines) 201 arranged in a grid pattern in a face-side surface 20 a, andoptical devices 202 such as light emitting diodes, laser diodes or thelike are formed in the thus demarcated regions.

Now, description will be made of a laser machining process in which theoptical device wafer 20 is irradiated with a laser beam along theplanned dividing lines 201 by use of the above-described laser beammachining apparatus, whereby a denatured layer is formed in the insideof the optical device wafer 20 along the streets 201. Incidentally, informing the denatured layer in the inside of the optical device wafer20, if a dispersion is present in the thickness of the optical devicewafer 20, it would be impossible to form the denatured layer uniformlyat a predetermined depth, due to a problem associated with therefractive index of the wafer, as above-mentioned. In view of this,prior to the laser beam machining, the height position of the opticaldevice wafer 20 held on the chuck table 36 is measured by use of theabove-described height position detector 8. Specifically, first, theoptical device wafer 20 is placed, with its back-side surface 20 b up,on the chuck table 36 of the laser beam machining apparatus shown inFIG. 1, and the optical device wafer 20 is suction held on the chucktable 36. The chuck table 36 with the optical device wafer 20 suctionheld thereon is positioned into a position just under the image pickupmeans 9 by the machining feeding means 37.

After the chuck table 36 is positioned in the position just under theimage pickup means 9, an alignment operation for detecting a machiningregion, to be subjected to laser beam machining, of the optical devicewafer 20 is performed by use of the image pickup means 9 and the controlmeans 10. Specifically, the image pickup means 9 and the control means10 carry out alignment by performing image processing such as patternmatching for position matching between the street 201 formed in theoptical device wafer 20 in a predetermined direction and the lightcondenser 7 of the height position detector 8 for detecting the heightof the optical device wafer 20 along the street 201. In addition,alignment is similarly carried out also with regard to the street 201formed in the optical device wafer 20 in a direction orthogonal to thepredetermined direction. In this case, the face-side surface 20 aprovided with the streets 201 of the optical device wafer 20 is locatedon the lower side. However, since the image pickup means 9 has imagepickup means comprised of the IR illuminating means, the optical systemfor catching the infrared rays, the image pickup device (IR CCD) foroutputting an electrical signal corresponding to the infrared rays thuscaught, etc., the image of the streets 201 can be picked up in asee-through manner from the side of the back-side surface 20 b.

After the alignment is carried out as above, the optical device wafer 20on the chuck table 36 is in the state of positioned in the coordinateposition shown in FIG. 10A. Incidentally, FIG. 10B shows the conditionobtained upon rotating the chuck table 36, namely, the optical devicewafer 20 by 90 degrees from the condition shown in FIG. 10A.

Incidentally, feed start position coordinate values (A1, A2, A3 . . .An) and feed finish position coordinate values (B1, B2, B3 . . . Bn) andfeed start position coordinate values (C1, C2, C3 . . . Cn) and feedfinish position coordinate values (D1, D2, D3 . . . Dn) of the streets201 formed in the optical device wafer 20 in the state of beingpositioned in the coordinate positions shown in FIGS. 10A and 10B arestored in the second storage region 103 b in the random access memory(RAM) 103.

After the streets 201 formed in the optical device wafer 20 held on thechuck table 36 are detected and alignment for the height detectionposition is carried out as above-mentioned, the chuck table 36 is movedso that the street 201 at the uppermost position in FIG. 10A ispositioned into a position just under the light condenser 7. Then, asshown in FIG. 11 further, the feed start position coordinate value (A1)(see FIG. 10A) which is one end (the left end in FIG. 11) of the street201 is positioned into the position just under the light condenser 7.Subsequently, the height position detecting means 8 is operated, and thechuck table 36 is moved in the direction indicated by arrow X1 in FIG.11 to the feed finish position coordinate value (B1) (height positiondetecting step). As a result, the height position (the distance (H) fromthe condenser lens 72 to the upper surface of the work W) at the street201 at the uppermost position in FIG. 10A of the optical device wafer 20can be detected as above-mentioned. The thus detected height position(the distance (H) from the condenser lens 72 to the upper surface of thework W) is stored into the third storage region 103 c in the randomaccess memory (RAM) 103, in correspondence with the coordinate valuesstored in the second storage region 103 b. The height position detectingstep is carried out in this manner along all the streets 201 formed inthe optical device wafer 20, and the height positions at the streets 201are stored into the third storage region 103 c in the random accessmemory (RAM) 103.

After the height position detecting step is conducted along all thestreets 201 formed in the optical device wafer 20 in this manner, lasermachining for forming a denatured layer in the inside of the opticaldevice wafer 20 along the streets 201 is performed. In carrying out thelaser beam machining, first, the chuck table 36 is moved so that thestreet 201 at the uppermost position in FIG. 10A is positioned into aposition just under the light condenser 7. Then, as shown in FIG. 12Afurther, the feed start position coordinate value (A1) (see FIG. 10A)which is one end (the left end in FIG. 12A) of the street 201 ispositioned into the position just under the light condenser 7. Thecontrol means 10 operates the light condensing point position adjustingmeans 53 so that the light condensing point Pa of the machining pulsedlaser beam LB1 radiated through the light condenser 7 is adjusted to aposition of a predetermined depth from the back-side surface 20 b (uppersurface) of the optical device wafer 20. Next, the control means 10operates the machining pulsed laser beam oscillating means 6 so that thechuck table 36 is moved in the direction indicated by arrow X1 at apredetermined machining feed rate while conducting irradiation with themachining pulsed laser beam LB1 through the light condenser 7 (machiningstep).

Thereafter, when the irradiation position of the light condenser 7 hasreached the other end (the right end in FIG. 12B) of the street 201,irradiation with the pulsed laser beam is stopped, and movement of thechuck table 36 is stopped. In this machining step, the control means 10controls the pulse motor 532 of the light condensing point positionadjusting means 53, based on the height position corresponding to theX-coordinate value at the street 201 of the optical device wafer 20stored in the third storage region 103 c in the random access memory(RAM) 103, whereby the light condenser 7 is moved in the verticaldirection in correspondence with the height position at the street 201of the optical device wafer 20, as shown in FIG. 12B. As a result, inthe inside of the optical device wafer 20, the denatured layer 210 isformed at the position of a predetermined depth from the back-sidesurface 20 b (upper surface) and in parallel to the back-side surface 20b (upper surface), as shown in FIG. 12B.

Incidentally, the machining conditions in the above-mentioned machiningstep are set, for example, as follows.

Laser: YVO4 pulsed laser

Wavelength: 1064 nm

Repetition frequency: 100 kHz

Pulse output: 2.5 μJ

Condensed spot diameter: φ1 μm

Machining feed rate: 100 mm/sec

Incidentally, in the case where the optical device wafer 20 has a largethickness, a plurality of denatured layers 210 a, 210 b, 210 c aredesirably formed, as shown in FIG. 13, by repeating the above-mentionedmachining step while changing stepwise the light condensing point Pa.The formation of the denatured layers 210 a, 210 b, 210 c is preferablycarried out by displacing the light condensing point of the laser beamstepwise in the order of 210 a, 210 b, and 210 c.

After the above-mentioned machining step is performed along all thestreets 201 extending in the predetermined direction of the opticaldevice wafer 20 in the above-mentioned manner, the chuck table 36 isturned by 90 degrees, and the machining step is carried out along eachof the streets extending in the direction orthogonal to thepredetermined direction. After the machining step is thus conductedalong all the streets 201 formed in the optical device wafer 20, thechuck table 36 with the optical device wafer 20 held thereon is returnedto the position at which the optical device wafer 20 has first beensuction held, where the suction holding of the optical device wafer 20is canceled. Then, the optical device wafer 20 is fed to a dividing stepby feeding means (not shown).

While an example in which the height position detector for a work heldon a chuck table based on the present invention is applied to a laserbeam machining apparatus has been shown above, the invention isapplicable to various machining apparatuses for machining a work held ona chuck table.

The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

1. A height position detector for a work held on a chuck table, fordetecting the height position of an upper surface of a work held on achuck table, comprising: laser beam oscillating means for oscillating alaser beam; annular spot forming means by which a spot of said laserbeam oscillated by said laser beam oscillating means is formed into anannular shape; a first beam splitter by which said laser beam with thespot formed into said annular shape by said annular spot forming meansis guided into a first path; a light condenser by which said laser beamguided into said first path is condensed so as to irradiate said workheld on said chuck table therewith; a pinhole mask disposed in a secondpath into which said laser beam reflected by said work held on saidchuck table is split by said first beam splitter; a second beam splitterby which said reflected light having passed through said pinhole mask issplit into a third path and a fourth path; a first light receivingelement for receiving said reflected light split into said third path bysaid second beam splitter; a second light receiving element forreceiving said reflected light split into said fourth path by saidsecond beam splitter; light reception region restricting means which isdisposed in said fourth path and which restricts a reflected lightreception region where said reflected light is received by said secondlight receiving element; and control means by which the height positionof said upper surface of said work held on said chuck table isdetermined based on the ratio between the quantity of light received bysaid first light receiving element and the quantity of light received bysaid second light receiving element.
 2. A height position detector for awork held on a chuck table as set forth in claim 1, wherein said annularspot forming means includes a pair of conical lenses arranged in seriesat a predetermined interval along said laser beam.